The AR-pattern firearm is among the most popular designs. These include configurations and variants, such as the AR15, AR10, M16, M4, and other variants. The AR-pattern lower receiver features a threaded socket and anti-rotation pocket at the rear where a “buffer tube” or “receiver extension” attaches and contains the recoil spring and buffer. This tube or extension is usually used to support or attach a stock for a rifle configuration or may support an arm brace in a pistol configuration. More recently, accessories and alternate upper receiver designs have appeared that relocate the action spring so the fixed extension or buffer tube at the rear of the receiver is no longer mechanically necessary. AR-pattern accessories that attach to the buffer tube, such as stocks and braces, have likewise become so popular that the threaded socket and anti-rotation pocket used on the AR-pattern have been included on many other types of firearms that do not mechanically require “buffer tubes.”
Accordingly, rifle stocks and pistol braces have appeared that replace these non-functional “buffer tubes,” adding capabilities, such as folding mechanisms not previously possible for the simple fixed tube. A number of these alternatives have even changed the mounting method at the receiver from the threaded socket of the original AR-pattern socket to a modular rail arrangement similar to that commonly used to mount sighting systems on these firearms. Specifically, these are a “Picatinny Rail,” also known as a MIL-STD-1913 or NATO Standardization Agreement 2324 rail, though other standardized rail formats could also be utilized in a similar fashion. At this time, these new rail-mounted accessories cannot be mounted to a standard AR-pattern receiver or other firearms that utilize the same “buffer tube” threaded socket mounting interface, even those where a buffer tube is not mechanically necessary.